As a system for suppressing background noise of an input signal having a plurality of sound sources each of which is configured of desired sound and background noise, a noise suppression system (hereinafter, referred to as a noise suppressor) is known. The noise suppressor is a system for suppressing noise superposed upon a desired sound signal. The noise suppressor, as a rule, estimates a power spectrum of a noise component by employing an input signal converted in a frequency region, and subtracts the estimated power spectrum of the noise component from the input signal. With this, the noise coexisting in the desired sound signal is suppressed. In addition, these noise suppressors are applied also for the suppression of non-constant noise by successively estimating the power spectrum of the noise component. There exists, for example, the technique described in Patent document 1 as a prior art related to these noise suppressors (hereinafter, referred to as a first related prior art).
Normally, the noise suppressor of the first related prior art, which is utilized for communication, fulfils a function as a pretreatment of an encoder. An output of the noise suppressor is encoded, and is transmitted to a communication path. In a receiving unit, the signal is decoded, and an audible signal is generated. The noise suppressor of the first related prior art is a one-input noise suppression system, in which, as a rule, residual noise that stays as a result of being not suppressed, and distortion of emphasized sound that is outputted are in a relation of trade-off. Reducing the residual noise leads to an increase in the distortion, and reducing the distortion leads to an increase in the residual noise. The best status of a balance between the residual noise and the distortion differs dependent upon individual users. However, with a configuration in which the noise suppressor exists in the upstream side of the encoder, namely, exists in a transmission unit, the user cannot adjust a balance between the residual noise and the distortion to its own taste.
As a noise suppressor assuming a configuration capable of solving this problem, a receiving side noise suppressor shown in FIG. 40 disclosed in Non-patent document 1 is known (hereinafter, referred to as a second related prior art). In the configuration of the second related prior art, a noise suppression unit 9501 is included not in the transmission unit, but in the receiving unit. The noise suppression unit 9501 performs a process of suppressing the noise of the signal inputted from a decoder. This enables the user to adjust a balance between the residual noise and the distortion to its own taste.    Patent document 1: JP-P2002-204175A    Non-patent document 1: IEEE INTERNATIONAL CONFERENCE ON CONSUMER ELECTRONICS, 6.1-4, January 2007